The idea of using vehicle position information obtained by means of a GNSS navigation satellite system to determine a toll amount is well known and in fact already applied operationally in some systems, although in combination with other technologies differing from GNSS. The basic concept consists of using vehicle P-T (position, time) data along with the geographic information of an infrastructure subject to charge so as to determine, given a toll rule or criterion, whether or not the vehicle has used the infrastructure, and if it has, the toll amount to be charged. Its implementation requires an onboard device or OBU (onboard unit) including a GNSS receiver providing P-T data, and mobile equipment for data communication with a processing center.
In a generic manner, the infrastructure subject to charge can be a specific transportation road: highway, expressway or street, transportation roads within an area, a parking garage, etc. The charging criterion may also be a “fixed amount” type, i.e. a given amount is charged for road usage or for entering a geographic area delimited by a perimetral boundary within an established time period; or it can be a “variable amount” type, i.e. an amount is charged depending on the “amount” of usage that is made of said infrastructure. The “amount” of usage can be measured according to occupancy time in the infrastructure or according to the distance traveled therein.
In the “fixed amount” case, P-T data from the receiver is used to detect (yes/no) whether or not the vehicle has used the infrastructure subject to charge in the charging period established in the criterion.
The advantages of this concept or idea are undoubtedly enormous. On one hand, applying charges to any infrastructure does not require deploying costly equipment in roads and, yet even more interesting, the system is totally flexible when defining what is charged and how it is charged. It is therefore possible, for example, to implement a perimetral charging system for accessing large cities or to charge for the time parked in said perimeter, in the latter case eliminating traditional parking meters. In the case of highways and expressways the system provides the possibility of charging according to usage (kilometers or any desired combination of the distance traveled, time used, trajectory speed, stops, etc.) thereof without needing to install any toll infrastructure.
That is, the GNSS-based road charging system determines whether or not the infrastructure has been used and, therefore, whether or not an amount is to be demanded from the carrier of the onboard receiver or OBU; to that end there are two essential parameters relating to the road charging system:                Charging availability: Probability that a vehicle that has indeed used the infrastructure within the charging period is detected by the system and, therefore, charged. This parameter is essential so that it is acceptable to the public or private infrastructure operator.        Probability of mischarging: Probability that a vehicle carrying the onboard receiver or OBU who has not used the infrastructure during the charging period is wrongfully detected by the system and, therefore, mistakenly charged. This parameter is essential for potential users and for system credibility and viability, because:        on one hand it allows having prior guarantees that allow confronting refusal or wrongful claims from users who have used the infrastructure but refuse to pay; and,        on the other hand it allows limiting the number of justified claims from non-users who where mistakenly charged.        
Current GPS-based systems cannot guarantee minimum performance of the probability of mischarging parameter given that GPS-based position errors are not delimited, nor is the type of distribution known. It is important to stress that although GPS-based position precision is currently high, it does not assure that huge errors will not occur from time to time, and these errors could be translated into a mischarging. This means that someday when the number of vehicles equipped with an OBU increases and complexity of the network of highways in which road charging is applied becomes more complex (for example with neighboring highways having different rates), the number of mischarges will substantially increase.
However, as will be seen in the description of the present invention, the invention does allow delimiting the probability of mischarging parameter. To that end the present invention is based on the use of a GNSS receiver with guaranteed integrity such as, for example, the one defined and disclosed in European patent application EP 05076289.7, entitled “Method and System for Providing GNSS Navigation Position Solution with Guaranteed Integrity in Non-Controlled Environments”. In addition to providing position and time information, said onboard receiver/OBU with integrity guarantee provides the following additional data output:                A health flag (healthy/unhealthy). When the flag is healthy the position solution error in any one direction has an upper limit for this measurement that is an RPL (Radial Protection Level) amount with a probability equal to a known value called integrity of the position solution provided by the receiver IRX.        An RPL (Radial Protection Level), i.e. the amount which limits the error in the horizontal position according to a direction with a probability equal to IRx, i.e.:P(|{right arrow over (ε)}·{right arrow over (u)}|>RPL)=1−IRX  (1)wherein {right arrow over (ε)} is the position error vector and {right arrow over (u)} is any unit vector.        
It is important to note that RPL and HPL (Horizontal Protection Level), commonly known in civil aviation, are not exactly the same. HPL is the upper limit of the error modulus, whereas RPL is the upper limit in a specific direction. On the other hand, HPL is associated to an IRx probability value measured during a certain time period including several measurements, whereas RPL is defined for probability IRx associated to a single measurement.